6L6 into a st-70
- Thread starter c.granger
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goldear
Certifiable Audio Junkie
A 6l6 is pinout compatible with an EL-34 socket. But it does need to be biased differently. I think that a pair of 6l6s should be biased at about 100 to 110 mA (or 50 to 55 mA) per tube.
The bigger issue is that the plate load impedance of a 6L6 is not the same as that for an EL-34 (I can't remember off the top of my head if it is lower or higher, but I know that it is off by nearly 50%), so you will probably lower your power output a bit, becuase the windings on your OTs are designed for properly loading an EL-34.
Bottom line: Yes, this can be made to work; but your power output will be somewhat lower.
And actually one Stereo 70 Mod that was offered by Audio Research replaced the EL34s with 6L6s, but it did rebias the tubes. I actually have one of these that I build about 20 years ago. That was a fun little amp. :thmbsp:
The bigger issue is that the plate load impedance of a 6L6 is not the same as that for an EL-34 (I can't remember off the top of my head if it is lower or higher, but I know that it is off by nearly 50%), so you will probably lower your power output a bit, becuase the windings on your OTs are designed for properly loading an EL-34.
Bottom line: Yes, this can be made to work; but your power output will be somewhat lower.
And actually one Stereo 70 Mod that was offered by Audio Research replaced the EL34s with 6L6s, but it did rebias the tubes. I actually have one of these that I build about 20 years ago. That was a fun little amp. :thmbsp:
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goldear
Certifiable Audio Junkie
1.5 V on your Dyna test point will provide a quiescent current of about 50mA per tube. That should be a safe operating level, assuming that the stock bias circuit will let you achieve this level... Higher voltages mean a much greater chance of releasing the smoke, so be careful if you try this!Kegger said:
Kegger
R.I.P. 1/12/1966 - 6/1/2017
Thank you sir! I have the (3) 12at7 board from tubes4hifi with a bigger can cap and
more under chassis capacitance as well. It really sounds good but I like experimenting.
I've seen some use kt88's in there's as well, are they more riskey or about the same?
P.S. with mine now it's biased at the same meter reading as the original because of the resistors I used.
more under chassis capacitance as well. It really sounds good but I like experimenting.
I've seen some use kt88's in there's as well, are they more riskey or about the same?
P.S. with mine now it's biased at the same meter reading as the original because of the resistors I used.
goldear
Certifiable Audio Junkie
That bias voltage value I gave you is only valid for if you are using a shared 15.6 ohm cathode resistor (like the stock stereo 70 does). If you have changed this value, or configutation (two separate resistors), then this value will be incorrect.Kegger said:
Essentially the same applies to 6650's and KT 88s, except that you should run between 120 and 130 mA of bias current for a pair. This would give a value of between 1.8 volts or 2 volts on your test point on a Stereo 70.
NOSValves
Super Member
Actually this all depends on what version of a 6L6 is going to be used. The above advise on biasing would only hold true with a 6L6GC. If using any other version of a 6L6 they would never handle that kind of current. I personally would never use anything but a 6L6GC in the case of the ST70.
Craig
Craig
Kegger
R.I.P. 1/12/1966 - 6/1/2017
Agreed Craig for sure! And I was completely off on what resistors were on my tubes and
what I set my bias to. Don't know where my brain was.
Not saying that these tubes should be run, but below is I believe what my settings should be.
----------------
15 ohm bias resistor per tube with bias adjustments per tube.
40-45ma EL34
50-55ma 6l6GC
60-65ma 6550
Around 400-425v plate voltage on ST-70?
.6 devided by 15 = .040 which= 40 milliamps
.040 (40 milliamps) x 15 (bias resistor 15 ohm) = .6 (600mv meter reading)
.040 X 15 = .6 (600mv) gets you (40ma)
.050 x 15 = .75 (750mv) gets you (50ma)
.060 x 15 = .9 (900mv) gets you (60ma)
------------------------
what I set my bias to. Don't know where my brain was.
Not saying that these tubes should be run, but below is I believe what my settings should be.
----------------
15 ohm bias resistor per tube with bias adjustments per tube.
40-45ma EL34
50-55ma 6l6GC
60-65ma 6550
Around 400-425v plate voltage on ST-70?
.6 devided by 15 = .040 which= 40 milliamps
.040 (40 milliamps) x 15 (bias resistor 15 ohm) = .6 (600mv meter reading)
.040 X 15 = .6 (600mv) gets you (40ma)
.050 x 15 = .75 (750mv) gets you (50ma)
.060 x 15 = .9 (900mv) gets you (60ma)
------------------------
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soundmotor
super modified
c.granger said:
You'll be fine. Observe the biasing procedures as noted. I ran 6L6GC's in my ST70 for nearly 1 year w/o problems.
